Spamming is a lucrative business in spite of an extremely small response rate (as defined by the fraction of people who click on spam) because the cost of getting spam messages to users is so low. A key step for spammers in sending cheap spam messages is to use automated means to create millions of fake email accounts on services like Yahoo!. This involves solving CAPTCHAs—puzzles presented by Yahoo! and other Internet sites that are easy for humans to solve, but hard for automated systems to solve.
The term CAPTCHA is short for “Completely Automatic Public Turing Test to Tell Computers and Humans Apart.” CAPTCHA is a pattern recognition challenge (test) that a human can easily pass, but is difficult or impossible for a computer to pass. For example, humans are able to read distorted text, but computers cannot. As CAPTCHA systems on Yahoo! properties have become better at weeding out automated solvers, the spammers have taken to outsourcing just the CAPTCHA-solving job of the spamming process to human solvers. These human solvers, typically in third world countries, work from home or in sweatshops and represent a significant fraction of the CAPTCHA-solving traffic at Yahoo! today. Moreover, as the prices charged by “human farms” has been falling steadily, today's going rate for solving 1000 CAPTCHAs is approximately 70 cents. At this low price, we can expect to see steady growth in the human farm activity on Yahoo! systems.
Given that now so much of CAPTCHA solving work for spammers is being done by humans, it is all but impossible to design CAPTCHAs which are easy for legitimate human users and difficult for spammers. Hence the new emphasis in CAPTCHA design work is to make the CAPTCHA more expensive to solve; in particular in terms of how much time it takes to solve it. This will cause human farms to charge more per correctly solved CAPTCHAs, and hence the cost of spamming will increase (making it unsustainable with the current low response rate). Moreover, since legitimate users need to solve a CAPTCHA infrequently, these legitimate users will not mind solving a more time-consuming CAPTCHA.
One known solution to make CAPTCHA-solving more time consuming is to display only a small portion of the image at any one time with the rest occluded. While this works well initially, we expect spammers to attempt attacks on these CAPTCHAs by techniques like speeding up the animation or taking multiple snapshots.
Therefore, there is a need for an improved CAPTCHA-solving technique to thwart the human spammers.